1. Field of Invention
The present invention relates to an electrical brush that conducts electrical currents across interfaces between two electrically conducting members in relative motion to one of which the brush is rigidly mechanically connected and the other of which is called the substrate. The electrical brush can be of monolithic type, including a solid piece of graphite or a metal-graphite mixture, or can be a metal fiber brush that is substantially composed of conductive fibers that occupy a “packing fraction”, f, of typically about 15% of the fibrous part of a fiber brush, with the remaining volume fraction of (1-f) being voidage, which arrangement permits individual flexibility for the average conductive fibers.
2. Discussion of the Related Art
Sliding of an electrical brush for an extended period or distance on a substrate, while it conducts an electrical current across the interface between the brush and the substrate, requires the presence of a thin interfacial film that typically adheres both conductive members, i.e. to at least a part of the brush and to the substrate. This interfacial film is needed to prevent cold-welding between the brush and the substrate that would cause very high friction and very high wear rates. Further, in the only mode of operation previously known, according to the theory presented in, for example, “Metal Fiber Brushes” by D. Kuhlmann-Wilsdorf, Chapter 20 in “Electrical Contacts: Principles and Applications” (Ed. P. G. Slade, Marcel Dekker, NY, 1999, pp. 943-1017), load-bearing indirect (i.e. via the interfacial film) mechanical contact between an electrical brush and a substrate is restricted to “contact spots” that occupy only a very small fraction of the footprint of the average conductive member and even less (i.e. by the factor of f) of the macroscopic foot print of the brush on the substrate, the footprint being the macroscopic area of the substrate that is covered by the sliding area of the brush, respectively of the sliding conductive brush element.
As already indicated, load-bearing contact between brushes and substrate was, according to the best previous scientific understanding of metal fiber brushes, made via contact spots that normally were formed by surface asperities, typically one or a very few asperities on the average fiber tip that contact the substrate. According to previous usage and best understanding, virtually all of the current flowed through those contact spots, virtually all of the electrical resistance was concentrated at the contact spots, and virtually all friction and wear originated at the contact spots. Following eq.20.45 of the already cited chapter 20 in “Electrical Contacts”, for a metal fiber brush of footprint area AB, the total contact spot area through which the brush current flowed in the indicated previously only known mode of metal fiber brush operation, wasAC≈3×10−4β2/3fAB  (1a)wherein β is the fraction that the macroscopically applied mechanical brush pressure represents of that brush pressure at which the average contact spot is still elastically deformed but above which it is deformed plastically.
For satisfactory brush operation without unduly fast mechanical wear, in the mode of current conduction with contact spots, β is typically chosen between 0.3 and 0.5, but it may be as low as β=0.1 for conditioned brushes according to the present invention. Correspondingly, with f=15%, the area AC of current conduction at the interface, namely through contact spots, accounts for only2×10−5<˜AC/AB<˜2.8×10−5, i.e. AC≅2.5×10−5AB  (1b)of the macroscopic brush footprint area, AB, in the previously only known mode of metal fiber brush operation.
According to accepted previous theory, the above is true also for monolithic brushes, except that (i) for these monolithic brushesAC≈3×10−4AB  (1c)since β=1 and f=1, (ii) the monolithic brushes include only one electrically conductive member and (iii) the typical number of contact spots of monolithic brushes is on the order of ten as compared to about one contact spot per fiber end, i.e., many thousands for metal fiber brushes. Further, in the course of sliding and incidentally mechanically wearing, monolithic brushes deposit a thin, electrically conductive graphitic surface film on the substrate that is apparently composed of consolidated wear debris. Ordinarily, the relative motion between a monolithic brush and a substrate takes place in that graphitic surface film, wherein graphite serves as a lubricant.
Since metal fiber brushes do not contain graphite, metal fiber brushes do not form a graphitic surface film on the substrate. The properties of the typical insulating surface film at the interface between normally operating metal fiber brushes and their substrates that prevent cold-welding, have been extensively discussed in U.S. Pat. Nos. 4,358,699, 4,415,635, and 6,245,440 as well as in the already cited “Metal Fiber Brushes” by D. Kuhlmann-Wilsdorf, Chapter 20 in “Electrical Contacts: Principles and Applications” (Ed. P. G. Slade, Marcel Dekker, NY, 1999, pp. 943-1017), the entire contents of which is incorporated by reference herein, and several scientific publications referenced in these.
Therein, it has been explained that, to date, the almost exclusively-used surface film to prevent cold-welding of metal fiber brushes is simply adsorbed water that, fortuitously, in our surroundings and in humidified protective atmospheres, establishes itself automatically. This is also a very prevalent surface film that overlays the already discussed graphitic film and is present during operation of monolithic graphitic brushes, as the lubricating property of graphite depends on the presence of adsorbed moisture without which graphite can become abrasive
Moreover, in the previously only known mode of operation, at the contact spots in the interface between the substrate and metal fiber brushes as well as monolithic brushes, under normal operating conditions, the adsorbed moisture film squeezes out into a double mono-molecular layer of adsorbed water, one on each side, of a total thickness≅5 Å=0.5 nm. Normal operating conditions of metal fiber brushes involve, for example, brush pressures in the order of pB=104 N/m2 and speeds below v=100 m/sec, and the relative sliding takes place between the two adsorbed mono-molecular layers with a friction coefficient of μ≅0.34. Brush pressures for monolithic graphite or metal graphite brushes are typically rather higher and, for these pressures, relative sliding takes place mostly in a thicker layer of consolidated wear debris that is overlaid with the same adsorbed moisture film but is more shearable, for example at μ≅0.2, than the moisture film. However, on account of the greater mechanical stiffness of the monolithic brushes, the monolithic brushes do not slide successfully at velocities above, for example, 40 m/sec.
When metal fiber brushes are operated in an adequately humid atmosphere, e.g. the open air under most conditions or when the metal fiber brushes are operated in a technologically widely favored protective atmosphere of moist CO2, the interfacial film that separates the brush from the substrate and that prevents cold-welding is essentially the described double mono-molecular film of adsorbed water. It typically has a film resistivity of σF≅10−12 Ωm2 within a factor of two or so, and a friction coefficient of μ≅0.35 for sliding between the two layers already indicated. Various disturbances, presumably foremost among these adsorbed oxygen that competes with the adsorption of water molecules, may raise those values to between μ≅0.4 and 0.6 and σF≅3×10−12 Ωm2 within a factor of two or so, provided that the interface is free of disturbing insulating surface films, foremost among these surface films being oxide films, that can drastically raise σF and can change friction in either direction. Therefore, operation in the open atmosphere, so as to eliminate unacceptably thick insulating surface films, may require protecting the substrate surface by use of noble metals, e.g. applied in the form of a noble metal plating.
Commercial monolithic brushes sliding on copper or copper alloys typically do not employ special measures to protect the substrate from oxidation because the graphitic film that is deposited offers adequate protection. On the down-side, often the presence of a well-seasoned graphitic layer is needed for proper functioning of commercial monolithic brushes, and as it happens, such layers have a tendency to deteriorate in periods of rest and under any number of other influences. As a result, monolithic brushes may perform erratically and pose problems in high-tech applications, e.g. in the main motor-generator of submarines. Further, and in line with the preceding explanation, also monolithic brushes, unless specially formulated, require adsorbed moisture. This is for the reason that graphite is a layer-type crystal whose shearability depends on the presence of water and which in dry conditions is brittle and abrasive.